1. Field of the Invention
The present invention relates to an electroluminescent device comprising an oxadiazole compound or derivative serving as an electroluminescent material and/or as a charge transporting material, with excellent film-forming properties and electroluminescence with a satisfactorily high luminance.
The present invention also relates to novel oxadiazole compounds which are useful not only as organic electroluminescent materials and charge transporting materials, but also as fluorescent brightening agents, and to a method of producing the oxadiazole compounds.
2. Discussion of Background
In recent years, a demand for a planar display device with a smaller power consumption and a smaller space occupation in comparison with a cathode-ray tube (CRT) is increasing in accordance with the diversification of the development of information systems. As such a planar display device, for instance, liquid crystal displays and plasma displays are in general use. However, recently, attention has been paid to electroluminescent devices (EL device) of a light emission type, which are capable of displaying clear images.
The above-mentioned electroluminescent devices can be classified into inorganic electroluminescent devices and organic electroluminescent device, depending upon the electroluminescent material employed therein. Of these, inorganic electroluminescent devices have already been used in practice.
An inorganic electroluminescent device is of a so-called collision and excitation type and is driven and emits light by the collision of electrons accelerated by the application of a high voltage to a luminescent center. Because of the application of high voltage for driving the electroluminescent device, such an inorganic electroluminescent device has the problem that the cost of peripheral equipments therefor are expensive.
In contrast to this, an organic electroluminescent device of an injection luminescence type comprises an organic luminescent layer interposed between positive and negative electrodes, each with a different work function, in which positive holes injected from the positive electrode and electrons injected from the negative electrode are combined in the organic luminescent layer, and a light with the same wavelength as that of the fluorescence of the luminescent layer is emitted. Because of the injection luminescence, the organic electroluminescent device can be driven by the application of a low voltage and can emit a luminescent light with a different color as desired by changing the electroluminescent material employed in the organic luminescent layer.
In addition to the above, the properties of organic compounds employed in the above-mentioned organic electroluminescent device differ, for instance, in accordance with substituent groups contained therein, so that the desired electroluminescent material can be fabricated from organic materials with more freedom in comparison from inorganic materials.
Therefore, it is considered that any electroluminescent material can be obtained by changing the molecular structure of the organic compound employed, if the conditions of electrons in the molecule are taken into consideration. Theoretically, it is possible to emit lights with any colors including red through blue by use of organic compounds. A variety of electroluminescent materials capable of stably emitting light with a color such as green, yellow or orange are actually proposed.
Conventionally, the following electroluminescent devices with a two-layered structure and electroluminescent device with a three-layered structure have been reported: a two-layered electroluminescent device with an SH-A structure in which a hole-transporting layer and a luminescent layer are formed between a hole injection electrode and an electron injection electrode (Japanese Laid-Open Patent Application 59-194393 and Appl. Phys. Lett. 51,913 (1987), and a two-layered electroluminescent device with an SH-B structure in which a luminescent layer and an electron-transporting layer are formed between a hole injection layer and an electron injection layer (U.S. Pat. No. 5,085,947, Japanese Laid-Open Patent Application 2-25092, and Appl. Phys. Lett. 55,1489 (1989); and a three-layered electroluminescent device with a DH structure in which a hole-transporting layer, a luminescent layer, and an electron-transporting layer are formed between a hole injection electrode and an electron injection electrode (Appl. Phys. Lett. 57,531 (1990)).
As the material for the above-mentioned hole injection electrode, materials with a large work function such as gold and ITO (indium-tin-oxide) can be employed, and as the material for the electron injection electrode, materials with a small work function such as calcium, magnesium, aluminum, and alloys thereof can be used.
Moreover, varieties of organic compounds have been proposed for use in the hole-transporting layer, luminescent layer and electron transporting layer.
Examples of the above-mentioned organic compounds include aromatic tertiary amines for the hole-transporting layer; aluminum trisoxine (Japanese Laid-Open Patent Applications 59-194393 and 63-295695), styryl amine derivatives, styryl benzene derivatives and the like (Japanese Laid-Open Patent Application 2-209988) for the luminescent layer; and oxadiazole derivatives (Nippon Kagaku Kaishi No. 11, p.1540 (1991)), Japanese Laid-Open Patent Applications 4-212286, 4-308688, 4-363891 and 4-363894) for the electron-transporting layer.
Electroluminescent devices with various structures with an initial luminance of as high as 1000 cd/m.sup.2 or more by the application of a drive voltage of about 10 V have been fabricated by use of organic electroluminescent materials. However, when the above-mentioned electroluminescent devices comprising the conventional organic materials are continuously driven, the luminescent output reduces within several hours and the drive voltage therefor has to be increased.
Thus, the conventional EL devices have a serious problem in the durability when used for an extended period of time.
In particular, suitable luminescent materials for an electroluminescent device for the emission of blue light have not yet been discovered, so that there are many problems to be solved such as the improvement of luminescence efficiency in the blue-light emitting electroluminescent device. In addition to the above, the research and development of a carrier-injection type electro-luminescent device comprising an organic compound as an illuminant have been just started and materials for the electroluminescent device and the application thereof have not sufficiently been studied. As a matter of course, the carrier-injection type electroluminescent device has many problems to be solved such as the improvement of luminance and durability of the device, and the control of the wavelength of the light emitted therefrom.
Varieties of oxadiazole derivatives are conventionally known, and are effectively utilized as the luminescent components for conventional electroluminescent devices. For instance, Japanese Laid-Open Patent Application 3-205479 discloses an electroluminescent device comprising as a luminescent component an oxadiazole derivative, which includes an alkenyl group, a carbazolyl group or an aminophenyl group as a substituent. However, there are some problems in the luminance and film-forming properties of the oxadiazole derivative used in the above electroluminescent device.
As mentioned previously, no luminescent materials which can stably emit blue light with a high luminance have been developed either for inorganic EL devices for organic EL devices.
1,1,4,4-tetraphenyl-1,3-butadiene derivatives and styryl benzene derivatives have been proposed as luminescent materials capable of emitting blue light for use in an organic EL device. However, these materials have no satisfactory film-forming properties, and no satisfactory luminance and stability when used in the electroluminescent devices. Moreover, as an electron-transporting material, 2-(4-tert-butylphenyl)-5-(p-biphenyl)-1,3,4-oxadiazole is disclosed in Japanese Laid-Open Patent Application 2-250292, but this material is easily crystallized and lacks in the stability.
Furthermore, an oxadiazole compound represented by the following formula (A) is disclosed in Japanese Patent Publication 45-2467: ##STR1## wherein Ar is phenyl group, or 4-tert-butylphenyl group.
However, the above oxadiazole compound is merely employed as an intermediate for producing stilbene derivatives.